A method for preventing rendering of content at overlapping time periods on more rendering devices than permitted by a license associated with the content is disclosed. The method includes: transmitting the following to a rendering device of a user: the content, first software that is operative to receive the content and to associate keys with the content, identification information (ID) that is associated with the user, and second software that comprises a player for rendering the content, receiving the following information from the rendering device during rendering of the content: a number representation X which comprises a number representation in bits of a result obtained from encrypting together a number representation of a present time interval and the ID, both encrypted with a public key of a key pair generated at the rendering device, and a number representation which comprises a number representation in bits of a share (SH) of a private key of the key pair generated at the rendering device, and detecting an attempt to render the content at overlapping time periods on more rendering devices than permitted by the license associated with the content based on a determination that number representations X received from separate rendering devices are identical, and number representations Y received from the separate rendering devices and paired with the number representations X received from the separate rendering devices are different. Related apparatus and methods are also disclosed.